Why does my shower switch keep burning out?

Question

I have a Mira Shower 10.8 kW.

My switch is a 45A switch. I have a 50A fuse for the shower. The shower uses 10mm² cable.

9 months ago the switch sparked and refused to turn off and the fuse tripped. Opening up the switch, the cables were black and burnt. I replaced the switch. 9 months on and it's happened again.

I know you're probably supposed to cut the cable and expose copper but I have no length left so I'll have to clean it with a wire brush and replace the switch.

Why does this keep happening though?

Answer 1

Mira seems to be based in the UK, so I'm assuming a 230V electrical mains.

Just taking the nominal figures for power and voltage gives: 10.8kW / 230V = 46.96A. If you look at the worst case mains voltage (230V - 10% = 207V), you could actually be drawing over 52A.

So, if your shower is drawing as much power as it's rated for (say if the incoming water is very cold, or it's a cold day and you've got the water temperature turned up), a 45A switch is actually underrated for the job it's doing. I'd recommend replacing it with a heftier switch.

Answer 2

Because it sparked, my guess is the switch you're using is not big enough. The pull-cord style double-pole switch at 45A is foreign to me (I have never seen anything like it in north america), but the specs on the Mira site say to use a switch with at least 3mm of contact separation.

This separation is important, because at these currents there is going to be arcing every time the power is toggled, which can do several things that negatively impact any type of switch/relay:

• "Burn" the terminal pads, which scorches them with the same black you see on the burnt wires outside, and over time, increases the resistance and thus increases the current draw through the pads, leading to even worse arcing
• Weld the terminals together

My guess with the info provided is both of the above happened in this case. The burn on the wires is probably caused by the arching inside the switch: maybe there is inadequate mechanical separation between the contact pads of the switch and the outside terminals?

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My inclination, if I were to ever install something like this here, is that 45A is too much for any type of user-facing switch that will be used on a regular basis. I would install a contactor (basically, a big relay) separate from the switch, which is rated at at least 60A (since as Nial C pointed out, depending on the exact voltage, you could be seeing as much as 52A). Then I'd run a separate circuit to the pull-switch (or whatever you're using) that simply controls the power to the contactor coil. This circuit would only have a minimal current in it (<1A) required to run the contactor coil.

Here's a quick diagram hacked together based on the Mira install manual:

Note though, I do not know if this would comply with EU plumbing/electrical codes! It seems a whole lot safer to me as it keeps a human from having to interact directly with a high current switch, and it's likely easier to replace the contactor if needed (though I'd guess it's less likely to be needed, as contactors are very commonly used in industrial systems to control high-current loads for many years without issue).

Answer 3

Seems strange to me to have a 45A switch and a 50A fuse (is that an MCB in the consumer unit?). What is protecting the switch from overcurrent? If you have a component that is rated to 45A I think it should be protected by at most a 45A fuse/MCB. If your 45A switch is only protected by a 50A fuse, then you could be drawing a dangerous overcurrent through the switch for too long before the fuse/MCB will detect a problem. So just on that basis I think you might need to either put in a smaller fuse/MCB or a higher rated switch (actually a higher rated switch, as your shower can draw more than 45A).

Another thing that is worth considering is the rating of the 10mm2 cable. Although 10mm2 cable can be used for (e.g.) 50A currents it is only rated that high if there are no derating elements to consider. For example if it is run inside insulation or next to other cables then its rating is lower. 10mm2 run by itself inside conduit in an insulated wall is only rated to 46A.

Personally I would seriously consider putting in a less powerful shower. I did that in my current house when I found that the cable and switches were not rated high enough. (the shower was pretty old and dodgy anyway so replacing it was not a big issue).

Another thing that might affect you is the size of the main fuse to your house. Some older houses only have a 60A supply.

Answer 4

If your cables are burnt it is not a faulty switch, it is poorly installed. The reason your cables are burnt is because heat has built up in the connection to the switch. The cable must be clean and securely clamped to ensure minimum resistance.

The formula for heat dissipation is I^2 * R. In your case, I = 45A. With just 0.1 ohms of resistance at the screw terminal you will create over 200 Watts of heat and the poor old terminal will not be able to get dissipate it. The end result is that it will burn the cable.

The fix is to clean the conductors and screw them down tightly.

Answer 5

I had the same problem with a 10.8kW Mira. Initially I used a 45A d/p pull switch, protected by a 63A Residual Current Device (RCD) and a 50A Miniature Circuit Breaker (MCB). The switch burned out.

I then purchased a 50 A d/p pull switch. Nine months later the switch burned out again and the 63A RCD had burned out. I used a 10mm twin and earth cable. Also the cable run was quite substantial, approximately 30 metres, so voltage drop could be an issue.

Your RMS or root mean square voltage in the UK does fluctuate between 220V and 250V so check that out and do a calculation to see how many amps you are actually drawing. (power over volts)